Biology 265 EVOLUTION

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Biology 265EVOLUTION

• Lecture 8

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Overview

• Problems for natural selection
• Ornaments and weapons
• Sexual selection
• Inter and intrasexual selection
• Fishers runaway process
• Handicap principle
• Classic evolutionary experiments

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What are they fighting about?
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The Drive to Replicate

• Life is not just about survival
• its about surviving long enough to reproduce
  successfully.
• Fitness is relative,
• and the fittest individuals are those that
  contribute the most genes to the next generation

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Fitness is relative, that means conflict

• Competition to fertilize or be fertilized

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Sexual SelectionDefined by Darwin (1859)
Sexual selection depends, not on a struggle for
existence, but on a struggle between the males
for possession of the females the result is not
death to the unsuccessful competitor, but few or
no offspring
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Intrasexual selection weapons

• Darwin (1859)
• Generally, the most vigorous males will leave
  the most progeny
• but in many cases, victory depends, not on
  general vigor, but on having special weapons,
  confined to the male sex.

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Male salmon - hooked jaw
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Stag beetle
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Irish Elk
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Predator defense or competition with other males?

• Barrette and Vandal (1990) studied sparring in
  caribou.
• Of 713 matches between males of different antler
  size, males with smaller antlers withdrew 90 of
  the time.

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Competition for mates

• Not surprising, nor does it pose deep theoretical
  questions

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Intersexual selection ornaments

• Darwin (1859)
• Amongst birds the contest is often more
  peaceful
• female birds, by selecting the most melodious
  or beautiful males might produce a marked
  effect.

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Also known as epigamic selection (usually female
choice)

• Lampyridae - fireflies
• bioluminescent
• flash lights to attract mates
• huge diversity
• species specific code

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Intersexual selection is complicated

• Raises some deep theoretical questions

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Darwin (1871)

• The Descent of Man, and Selection in Relation to
  Sex

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Comparative evidence for sexual selection

• Polygynous species one male mates with multiple
  females some males dont mate
• Selection for male to get access to females is
  strong - winner takes all!
• Consequently, predict more sexual dimorphism in
  polygynous than monogamous species

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Data supports hypothesis

• Darwin showed that polygynous species are more
  sexually dimorphic
• Also, the rarer polyandrous (one female mates
  with multiple males) species showed strong sexual
  dimorphism
• In polyandrous birds, such as pharalopes, females
  are large and bright and compete for males

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Different models of intersexual selection

• Female choice co-evolved with trait exaggeration
  (Fisher's process)
• Females that are choosy gain direct benefits
• Females are choosy because of sensory bias

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R.A. Fisher (1930) Genetical Theory of Natural
Selection

• Initially, female preferences evolve because the
  preferred trait is favored by natural selection
• Consequently, males with the trait are even more
  fit (natural and sexual selection)
• Creates an ever increasing selective force
  favoring stronger preferences and more extreme
  traits - runaway process
• stops when no more variation or natural selection
  costs outweighs sexual selection advantages

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Evidence

• In a breeding experiment with the threespine
  stickleback
• Bakker (1993) observed a genetic correlation
  between red coloration among sons and preferences
  for red coloration among daughters
• as expected under the Fisherian process

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Drawback of Fisherian process

• If being choosy is costly to the female, it is
  unlikely to happen
• Choosiness is likely to be costly
• expend time and energy evaluating mates
• risk failure to mate

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Different models of intersexual selection

• Female choice co-evolved with trait exaggeration
  (Fisher's process)
• Females that are choosy gain direct benefits
• Females are choosy because of sensory bias

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Direct benefit of choosiness

• Lower risk of mating with the wrong species
  (species recognition)
• Mate may provide paternal care, territorial
  defense, food
• Mate may be more compatible, or less likely to
  pass diseases/parasites to female
• Offspring may be more fit (Good genes hypothesis)

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Evidence I

• Linden (1991)
• great tits, Parus major, avoid previous mates
  with whom they had few offspring
• compared with those with whom they had many
  offspring.

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Evidence II

• Thornhill (1983)
• female hangingflies
• have a higher egg laying rate when mated with
  males that provide large nuptial food gifts

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Handicap Principle (Is evolution really this
twisted?)

• Zahavi (1975)
• If males vary in the quality of their genes,
  females should choose carefully
• But choose on what basis?
• Males that can survive despite a costly ornament
  (handicap) must genuinely have good genes

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Evidence

• Mollers (1994) study of barn swallows
• Artificially increased tail length
• Males with longer tail attract mates more quickly
  and have more offspring
• But in the next year, their (real) tail grows
  back after the molt and is shorter than it was
  before - demonstrating the cost of flying for a
  year with a long tail

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Parasite hypothesis

• Hamilton and Zuk (1982)
• Females select males mainly for good genes
  providing resistance to parasites
• Bright plumage is a sign of health
• Moller found that males with longer tails had
  fewer mites

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Different models of intersexual selection

• Female choice co-evolved with trait exaggeration
  (Fisher's process)
• Females that are choosy gain direct benefits
• Females are choosy because of sensory bias

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Sensory Bias

• female preferences are simply a side effect (a
  "pleiotropic" effect) of sensory evolution which
  has evolved for other reasons.

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Evidence

• Searcy (1992)
• female common grackles preferred males singing an
  artificial repertoire with four song types
• even though males in this species sing only one
  song type

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Classic studies

• Sexual selection exemplifies evolutionary study
• Mechanism and preliminary evidence proposed by
  Darwin
• Hypotheses generated from data and theoretical
  models
• Predictions tested in well controlled field
  experiments